GLACIERS OP PRINCE WILLIAM SOUND 238 its mouth. The topography of Prince William Sound is wholly mountainous. The greatest elevations are in the interior of the Chugach and Kenai Ranges and the peninsulas which project into the sound slope gradually to moderate heights and then end abruptly. There is an exceedingly small percentage of lowland. All of the islands are hilly, and the larger ones, like Knight and Latouche Islands, are mountainous. The topography, geology, geography, and glaciation of Prince William Sound and the lower Copper River have been described by Schrader,1 Mendenhall,2 Schrader and Spencer,8 Gilbert,4 Emerson and Palache,6 Gannett,8 Burroughs,7 Muir,8 Keeler,* Fernow,10 Davidson,11 Grant and Higgins,12 and others whose work is referred to in Chapters XTU to XXHI. The geology of the Prince William Sound and lower Copper River regions may be summarized as follows. In Prince William Sound Grant1S states that there are two groups of rocks. The Valdez group is older, perhaps Paleozoic, and consists of slates and graywackes. These are intensely folded in places and are somewhat metamorphosed. They contain granitic intrusions. The unconformable Orca group, probably Mesozoic, is made up of slates, graywackes, and rarer conglomerates and limestones. Locally there is much greenstone, made by the alteration of basic lava flows which were inter-stratified with the slates and graywackes. These are cut by bosses and dikes of granite and by basic dikes. Northern and western Prince William Sound, including all of the fiords except Ports Fidalgo and Gravina and Orca Inlet, are underlain by the rocks of the Valdez group while the islands and eastern shore of the sound, from Ellamar southward, are made up of the Orca group. It seems possible that the excavation of this great arm of the sea is related to its being floored by the relatively less resistant rocks of the Orca group. In the lower Copper River, Hayes,14 Schrader and Spencer,15 Moffit and Maddren18 have described the geology. There are three rock groups, (1) the Mesozoic Orca group 1 Schrader, F. C., A Reconnaissance of a Fart of Prince "William Sound and the Copper Hirer District* Alaska, in 1898, 20th Ann. Kept., U. S. GeoL Survey, Part VH, 1900, pp. 847-423. i Mendenhall, W. C., A Reconnaissance from Resurrection Bay to the Tanana River, Alaska, in 1808, 20th Ann. Hept., U. S. Geol. Survey, Part VH, 1900, pp. 271-840. i Schrader, F. C. and Spencer, A. C., The Geology and Mineral Resources of a Portion of the Copper River District, Alaska, House Doc. 546, 56th Congress, 2nd Session, Washington, 1900, pp. 9-92. • Gilbert, G. K., Harriman Alaska Expedition, Vol. IH, Glaciers, 1904, pp. 71-97, 173-176. i Emerson, B. K. and Palache, Charles, Harriman Alaska Expedition, Vol. n, Geology, 1904, pp. 24-26, 50-51. • Gannett, Henry, Harriman Alaska Expedition, Vol. II, 1900, pp. 262-263; Nat. Geog. Mag., Vol. X, 1899, pp. 510-n612; Bull. Amer. Geog. Soc., Vol. XXXI, 1899, pp. 846-348, 354-855. ' Burroughs, John, Harriman Alaska Expedition, Vol. 1,1901, pp. 63-76. • Muir, John, Ibid., Vol. I, 1901, pp. 125-127, 132-133. • Keeler, Charles, Ibid., Vol. H, 1901, pp, 219-221. » Fernow, B. E., Ibid., Vol. IX 1901, pp. 244, 248,263. « Davidson, George, The Glaciers of Alaska that are Shown on Russian Charts or Mentioned in Older Narratives, Trans, and Proc. Geog. Soc., Pacific, 2nd series. Vol. 3,1904, « Grant, U. S. and Higgins, D. F., Reconnaissance of the Geology and Mineral Resources of Prince William Sound, Alaska, Bull. 448, U. S. Geol. Survey, 1910; Tidewater Glaciers of Prince William Sound and Kenai Peninsula, Bull. 17. S. Geol. Survey (in press). » Op. cit. p. 11. » Hayes, C. W., Nat. Geog. Mag., Vol. IV, 1892, pp. 126,141-142,154. it Schrader and Spencer, op. cit. « Moffit, F. H. and Maddren, A. G., Bull. 874, U. S. Geol. Survey, 1909, PI. I.